The field of the invention is that of ad hoc communications networks.
Ad hoc communications networks use the radio medium. They consist of mobile and/or fixed nodes that have the property of automatically and dynamically constituting a network capable of routing packets from any point of the network to any other point of the network after radio communication is set up between a node and its neighbors.
In an ad hoc network, packets are transmitted between the source node and the destination node either directly if the destination node is in the connectivity area of the source node or via intermediate neighbor nodes if the destination node is out of range of the source node.
Consequently, ad hoc networks enable instantaneous deployment of communications networks with no pre-existing infrastructure and no centralized control. The network is formed dynamically, all management tasks being divided between all nodes of the network.
The main feature of ad hoc networks is that the nodes of the network serve or can serve as routers. The nodes themselves are therefore responsible for setting up and maintaining continuous network connectivity using specific routing protocols that enable exchange of routing information between neighbor nodes and enable the nodes of the network to calculate communication paths to all the other nodes. These routing protocols send messages periodically to update the topology of the ad hoc network, i.e. to identify the nodes and the links between them.
To be more precise, the invention relates to a routing method of transmitting data packets between a source node and a target node of an ad hoc network formed by mobile nodes moving on traffic routes of a particular geographical network between which intersections are formed, the nodes being used as carriers for routing data packets to the target node.
The invention finds a special but non-limiting use in vehicular ad hoc networks (VANET) and in urban environments.
Urban vehicular ad hoc networks are distributed and flexible intervehicular communication (IVC) wireless communications systems in which the communication nodes are vehicles moving along routes of a road network of known structure.
In such systems, each vehicle is equipped with short-range wireless transmission means enabling it to send and receive radio-frequency signals and thus to form, in conjunction with other vehicles, a temporary communications network.
In these networks, data packets are routed from a source vehicle to a target vehicle using a routing protocol whereby the data packets are successively forwarded by one or more vehicles moving on the routes of the road network between the source vehicle and the target vehicle.
Most existing routing protocols are based on geographical routing that exploits local information concerning the explicit geographical position of the nodes of the network in order to take decisions regarding transfer of data packets.
A first routing protocol known as the GSR (Geographic Source Routing) protocol bases routing on the geographical position of the nodes of the ad hoc network correlated with information relating to the topology of the network.
According to the GSR protocol, a source vehicle seeking to send a data packet to a target vehicle calculates the shortest routing path to reach the target vehicle based on geographical information from a road map. Note that the routing path in question is calculated in its entirety, for example using the Djikstra algorithm.
On the basis of the calculated routing path, the source vehicle then selects a sequence of intersections through which the data packet must pass in transit to reach the target vehicle. This set of intersections consists of a set of fixed geographical points through which the data packet is to pass.
Thus a complete sequence of fixed geographical points must be calculated before sending each packet to a target vehicle, this sequence then being inserted into the header of a packet to be sent to the target vehicle.
A drawback of the GSR protocol is that the sequence of intersections selected can include roads on which there are too few vehicles to provide a good connection, which has the harmful effect of increasing the packet loss rate.
The insertion of this kind of sequence represents an additional load on the ad hoc network, given that it must be inserted into the header of every packet sent to a target vehicle. Consequently, this kind of approach is disadvantageous in that it cannot optimize the bandwidth allocated to the ad hoc network.
Moreover, it should be noted that the step of determining the entire routing path for reaching the target vehicle is a greedy operation in terms of computation resources. This operation being carried out only at the source node, the GSR protocol does not provide for tracking the evolution of a moving target vehicle.
This can induce a relatively long latency time between a source vehicle sending a packet and a target vehicle receiving it.
When the routing path has been established, the data packet is forwarded successively from node to node along that path using a forwarding strategy known as “Greedy Forwarding”.
According to this forwarding strategy, a carrier node of a packet always seeks to forward that packet to a neighbor node that is nearer the target vehicle than the carrier node itself. As soon as the data packet is received, the neighbor node in turn becomes a carrier node that in turn seeks to forward the packet to a neighbor node, and so on.
However, this forwarding strategy can fail in the particular situation where no neighbor node of the carrier node is nearer the target node than the carrier node itself. This situation is known as a “local optimum” situation and requires a recovery solution.
The A-STAR (Anchor-based Street and Traffic Aware Routing) protocol is specifically designed for intervehicular communications networks in urban environments and differs from the GSR protocol in that it takes road traffic data into account when it calculates a routing path. This traffic data is static (for example based on statistics), however, and is not suitable for an urban network in which road traffic is constantly evolving over time. In particular, existing routing protocols in the context of intervehicular communication do not at present take into account space and time variations in road traffic density or the presence of multidirectional routes.